The preparation by a chemical ceramic sol-gel process of dense, alumina-based ceramic abrasive grain by gelling alumina monohydrate with a precursor of at least one modifying component followed by dehydration and firing is disclosed in assignee's U.S. Pat. No. 4,314,827 (Leitheiser et al). It was subsequently discovered that such ceramic abrasive grains may be improved by including a seeding material with the alumina hydrate to provide a finer crystalline structure in the fired alpha alumina. The seeding of alpha alumina ceramic obtained by firing alumina hydrate with alpha alumina seed material is known. M. Kumagai and G. L. Messing spoke at the American Ceramic Society meeting on May 2, 1984 on this subject and later caused the publishing of a paper in November, 1984 in Communications of The American Ceramic Society entitled "Enhanced Densification of Boehmite Sol-Gels by Alpha Alumina Seeding". U.S. Pat. No. 4,623,364 (Cottringer et al) discloses a method for producing polycrystalline alumina based abrasive grain by mixing submicron alpha alumina seed material with fine alumina hydrate particles, and firing to a temperature of at least 1090.degree. C. European Patent Application No. 209,084, published Jan. 21, 1987, discloses a method for producing a sol-gel type ceramic abrasive grain wherein a precursor stage material consisting of aluminum hydroxide is mixed, dried, calcined and sintered with an advanced precursor stage material, followed by dehydrating, calcining and sintering. Assignee's U.S. application Ser. No. 15,583 (Gerk) filed Feb. 17, 1987 discloses a method for producing a sol-gel type ceramic abrasive grain by adding fine particles of alpha alumina to the alpha alumina monohydrate prior to gelling and firing.
Assignee's U.S. Pat. No. 4,744,802 (Schwabel) discloses nucleating alpha alumina monohydrate with particles of alpha alumina, iron oxide or their respective precursors to produce alpha alumina ceramic abrasive grits. While the Schwabel disclosure indicates that the particle size of the nucleating agent can vary considerably, it discloses that, in the case of alpha alumina, a particle size from about 80 to about 100 nm was found useful but smaller and larger particles were also thought to be useful. The Schwabel patent discloses nucleating with ferric nitrate (iron oxide precursor) solution which transformed to particulate crystalline iron oxide particles in situ of an unknown particle size during processing prior to sintering and particulate alpha ferric oxide having a particle size of 0.2.times.0.02 micrometer (200.times.20 nm). Schwabel makes no mention that an unexpected performance improvement could result from seeding with particulate crystalline iron oxide having a particle size of less than 150 nm.
James L. McArdle and Gary L. Messing, in an article entitled "Transformation and Microstructure Control in Boehmite-Derived Alumina by Ferric Oxide Seeding" in Advanced Ceramic Materials, Volume 3, No. 4, 1988, disclose nucleating alumina gels with particulate ferric oxide to enhance the transformation from gamma alumina to alpha alumina. The nucleating agents include either alpha alumina particles or alpha ferric oxide particles having a particle size distribution of 15 to 90 nm with 80% of the particles being between 30 and 70 nm. The article does not, however, suggest that abrasive grain with unexpected improvement in performance may be obtained from utilizing crystalline iron oxide nucleating particles of an average particle size less than 150 nm.